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I'm a refrigeration guy who has never worked on a residential heat pump. However I am closing on a house that has an air to air heat pump with a gas furnace back up. Can anyone tell me if the sequence of events that is supposed to happen when the thermostats call for back up heat.

I know that with electric heat the two systems can operate together because the electric heat is downstream of the evaporator, but I read that with gas furnace back up heat the 2 should never operate at the same time.

I haven't closed on the hose yet so I cannot be there to check things, but I did notice a lot of frost on the outdoor coil when I was there for the building inspection.

It was a cool rainy day. I have been told that there will be no defrost cycle when the heat pump operates in conjunction with a gas furnace, because that would cause high head pressures as the evaporator absorbed heat from the furnace.

I also know that the AC coil is downstream of the heat exchanger and that is the correct way to do it. Otherwise cold air could cause condensation in the heat exchanger and ruin it.